In an optical device fabrication process, an optical device layer, e.g., composed of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer, is formed on the front side of a single crystal substrate, such as a sapphire substrate, a silicon carbide (SiC) substrate or a gallium nitride (GaN) substrate. The optical device layer is partitioned by crossing division lines (also referred to as “streets”) to define separate regions where optical devices, such as light emitting diodes (LEDs) and laser diodes, are respectively formed. By providing the optical device layer on the front side of the single crystal substrate, an optical device wafer is formed. The optical device wafer is separated, e.g., cut, along the division lines to divide the separate regions where the optical devices are formed, thereby obtaining the individual optical devices as chips or dies.
As a method of dividing a wafer, such as an optical device wafer, along the division lines, there has been proposed a laser processing method of applying a pulsed laser beam, having a wavelength allowing transmission of the beam through the wafer, to the wafer along the division lines in a condition where a focal point of the pulsed laser beam is located inside the wafer in a subject area to be divided. In this way, a modified layer having a reduced strength is continuously formed inside the wafer along each division line. Subsequently, an external force is applied to the wafer along each division line by using a breaking tool, thereby dividing the wafer into the individual optical devices. Such a method is disclosed in JP-A-3408805.
As another method of dividing a wafer, such as an optical device wafer, along the division lines, it has been proposed to apply a pulsed laser beam to the wafer in a condition where a focal point of the beam is located at a distance from the front side of the wafer in the direction towards the back side thereof, in order to create a plurality of hole regions in the single crystal substrate. Each hole region is composed of an amorphous region and a space in the amorphous region open to the front side of the wafer. Subsequently, an external force is applied to the wafer along each division line by using a breaking tool, thus dividing the wafer into the individual optical devices.
However, when applying the external force to the wafer using the breaking tool in the above-mentioned dividing methods, a shift of the resultant chips or dies relative to each other may occur. Such a die shift not only renders the process of picking up the chips or dies more complicated but also creates the risk of damage to the chips or dies, e.g., if their side surfaces touch each other due to the shift.
Further, the individual chips or dies may not be properly separated from each other by the application of the external force using the breaking tool. For one thing, two or more of the chips or dies may still be, at least partially, connected to each other after the breaking process, so that it is necessary to inspect the wafer after die separation. For another thing, the outer shape of the resultant chips or dies, i.e., the shape of their side surfaces, after separation thereof cannot be controlled with a high degree of accuracy.
The problems referred to above are particularly pronounced for transparent crystal materials which are difficult to process, such as silicon (Si), gallium arsenide (GaAs), gallium nitride (GaN), gallium phosphide (GaP), indium arsenide (InAs), indium phosphide (InP), silicon carbide (SiC), silicon nitride (SiN), lithium tantalate (LT), lithium niobate (LN), sapphire (Al2O3), aluminium nitride (AlN), silicon oxide (SiO2) or the like.
Hence, there remains a need for a method of processing a substrate which allows for the substrate to be processed in an accurate, reliable and efficient manner.